In the drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The lithography technology has achieved formation of finer patterns by a reduction of light source wavelength and a proper choice of a resist composition compliant therewith. The mainstream resides in positive photoresist compositions which are used as a single layer. The single layer positive photoresist composition contains a base resin with a skeleton having resistance to etching with chlorine or fluorine base gas plasma and is provided with such a resist mechanism that exposed areas are dissolved in developer. Briefly stated, the photoresist composition is coated on a patternable substrate, a pattern is formed by dissolving away the exposed areas, and the substrate is then etched with the remaining resist pattern serving as an etching mask.
Regrettably, in an attempt to achieve miniaturization, i.e., to reduce the pattern width, with the thickness of the photoresist film kept unchanged, the photoresist film is reduced in resolution performance. In the subsequent development of the photoresist film with a liquid developer for patterning, the so-called aspect ratio becomes too high, resulting in pattern collapse. Thus, the thickness of photoresist film must be reduced before miniaturization can be accomplished.
On the other hand, the process of processing a patternable substrate is generally by dry etching of the substrate with a patterned photoresist film serving as an etching mask. However, since no etching technique capable of establishing full etching selectivity between the photoresist film and the patternable substrate is currently available, the resist film can be damaged during the processing of the substrate. That is, the resist film can be broken up during the processing of the substrate, prohibiting the faithful transfer of the resist pattern to the patternable substrate. Then, as the pattern feature size becomes finer, the resist material is required to have higher etching resistance.
As a result of reduction of the exposure wavelength, on the other hand, the resin used in photoresist compositions is required to have low light absorption at the exposure wavelength. In accordance with a transition from i-line to KrF and to ArF, the resin also encountered a transition from novolac resins to polyhydroxystyrene and to aliphatic polycyclic skeleton resins. Actually, the etching rate of resin under the above-mentioned etching conditions has accordingly become higher, indicating the tendency that current photoresist compositions with higher resolution rather exhibit lower etching resistance.
This results in the situation that a patternable substrate has to be etched using a thinner, less etching resistant photoresist film. There exists an urgent demand for a material and process capable of accommodating this processing step.
One approach for solving these problems is the multilayer resist process. The process involves interleaving an intermediate film having different etching selectivity from a photoresist film, i.e., an overcoat resist film, between the overcoat resist film and a patternable substrate, processing the overcoat resist film to form a pattern, dry etching the intermediate film using the overcoat resist pattern as an etching mask, thereby transferring the pattern to the intermediate film, and dry etching the patternable substrate using the intermediate film as an etching mask, thereby transferring the pattern to the substrate.
The bilayer resist process falling within the category of the multilayer resist process uses a silicon-containing resin as the overcoat resist material and a novolac resin as the intermediate film, for example, as disclosed in JP-A 6-95385. The silicon-containing resin exhibits good etching resistance to reactive etching with oxygen plasma, but is readily etched away if fluorine gas plasma is used. In turn, the novolac resin is readily etched away by reactive etching with oxygen gas plasma, but exhibits good etching resistance to etching with fluorine or chlorine gas plasma. Then, a novolac resin film is formed on a patternable substrate as the intermediate film, and a silicon-containing resin is used to form an overcoat resist film thereon. A pattern is then formed in the silicon-containing resist film by irradiation of energy radiation and post-treatment such as development. Using the patterned resist film as an etching mask, reactive etching with oxygen plasma is carried out to etch away those portions of the novolac resin where the resist has been removed in pattern, thereby transferring the pattern to the novolac film. Using the pattern transferred to the novolac film as an etching mask, the patternable substrate is dry etched with a fluorine or chlorine gas plasma for transferring the pattern to the substrate.
In the dry etching, a transferred pattern is obtained in a relatively satisfactory shape as long as the etching mask has sufficient etching resistance. Since such a problem as pattern collapse caused by friction by a developer during resist development is unlikely to occur, a pattern having a relatively high aspect ratio is obtainable. As a consequence, if the thickness of a resist film using novolac resin corresponds to the thickness of the intermediate film, even in the case of a micropattern which could not otherwise be directly formed due to pattern collapse during development because of the problem of its aspect ratio, the bilayer resist process ensures formation of a novolac resin pattern having a sufficient thickness to serve as an etching mask for the patternable substrate.
Also included in the multilayer resist process is the trilayer resist process which can be implemented using conventional resist compositions as used in the single layer resist process. In this process, an organic film of novolac resin or the like is formed on a patternable substrate as an undercoat resist film, a silicon-containing film such as spin-on-glass (SOG) is formed thereon as an intermediate film, and a conventional organic resist film is further formed thereon as an overcoat resist film. With respect to dry etching with fluorine gas plasma, the organic resist overcoat film provides a definite etching selectivity relative to the SOG. Then the resist pattern is transferred to the SOG film by dry etching with fluorine gas plasma. With this process, even when a resist composition which is difficult to form a pattern having a sufficient thickness to allow a patternable substrate to be directly processed therethrough, or a resist composition having insufficient etching resistance to allow a substrate to be processed therethrough is used, a pattern of novolac film having sufficient etching resistance for processing is obtainable like the bilayer resist process as long as the pattern can be transferred to the SOG film.
As discussed above, the trilayer resist process uses a silicon-containing film as the intermediate film. Exemplary of the intermediate film used are silicon-containing inorganic films by CVD, for example, SiO2 film (as disclosed in JP-A 7-183194) and SiON film (as disclosed in JP-A 7-181688), and films by spin-on deposition, for example, SOG film (as disclosed in JP-A 5-291208) and crosslinkable silsesquioxane film (as disclosed in JP-A 2005-520354). Additionally, a polysilane film (as disclosed in JP-A 11-60735) would be useful. Of these, the SiO2 and SiON films perform well as etching mask when underlying organic films are dry etched, but require special equipment for their deposition. In contrast, the SOG, crosslinkable silsesquioxane and polysilane films can be formed merely by spin coating and heating, offering a high process efficiency.
The applicable range of the multilayer resist process is not limited to the attempt of increasing the resolution limit of resist films. If an attempt is made to form a pattern using a single resist film in the situation where a patternable intermediate substrate has substantial steps as in the via-first process which is one of substrate working processes, a problem arises that precise focusing is impossible during resist exposure because of substantial differences in resist film thickness. In such a case, planarization is achieved by burying steps in a sacrificial coating, after which a resist film is formed thereon, and a pattern is formed in the resist. This inevitably poses the use of a multilayer resist process as mentioned above. See JP-A 2004-349572.
In implementing the trilayer resist process, the efficiency of the process can be increased by using as the intermediate film a silicon-containing film which can be formed by spin coating. Nevertheless, the silicon-containing films used in the art suffer from several problems.
For example, it is well known that when a resist pattern is formed by photolithography, exposure light is reflected by the substrate and the reflected light interferes with the incident light to give rise to the problem of standing waves. To form a fine pattern without edge roughness from a resist film, an antireflective coating must be added as an intermediate film. For the trilayer resist process, this means that an organic antireflective coating must be disposed between an overcoat resist film and a silicon-containing intermediate film. If such an organic antireflective coating is added, there is a need that the relevant coating be patterned using the overcoat resist film as an etching mask. During the dry etching, an additional load is imposed on the overcoat resist film. One approach used in the art in order to eliminate such load is to provide the silicon-containing intermediate film with a light-absorbing structure such as an aromatic structure (as disclosed in JP-A 2005-15779).
However, anthracene and analogous organic structures capable of efficient light absorption function to reduce the rate of dry etching with fluorine gas plasma, pointing away from the dry etching of a silicon-containing film without imposing any additional load to the resist film. It is undesirable to incorporate substantial amounts of such substituent groups.
While oxygen gas plasma is often used in processing the undercoat resist film using the silicon-containing intermediate film as an etching mask, the etching rate of the intermediate film in reactive etching with the oxygen gas plasma is preferably lower in order to increase the etching selectivity ratio between the intermediate film and the undercoat film. To this end, the intermediate film desirably has as high a silicon content as possible. However, in the design of providing a light-absorbing function skeleton to side chains, the side chains occupy a higher proportion at the expense of a silicon content.
It is disclosed in JP-A 2003-140352 that polysilanes can have a relatively high silicon content and become appropriate antireflective coating materials. As opposed to the silicon-oxygen-silicon bond which remains transparent down to light of considerably shorter wavelength, the silicon-silicon bond absorbs light of longer wavelength and is deemed desirable as the silicon-containing intermediate film having an antireflective function. However, under halogen etching conditions, the polysilanes underwent cleavage of silicon-silicon bonds in their backbone and converted to low molecular weight components. A phenomenon that film strength diminished and the etched pattern collapsed was observed.